Road surface friction coefficient estimating method, signal multiplex transmission method and signal multiplex transmission device

ABSTRACT

A method for directly and accurately estimating friction coefficient of a road surface independently from the slip rate is disclosed. The method measures tangential and vertical forces acting on an elastic body  3  of an elastic wheel  1 , and calculates the friction coefficient of a road surface based on the measured values of these forces and angular rate of the wheel  1.

TECHNICAL FIELD

[0001] The present invention relates to a method for accuratelyestimating friction coefficient of a road surface which coefficient isindispensable to a control for enhancing a performance of an anti-lockbrake system (herein after referred to as an “ABS”) or a tractioncontrol system.

BACKGROUND

[0002] In order to improve a performance of an ABS used in a vehicle, itis considered to be effective to control lock and unlock states in acondition where friction coefficient of a road surface be as large aspossible. The friction coefficient of the road surface depends on a sliprate of a tire/wheel assembly under a certain condition of the roadsurface and therefore the ABS is designed to control lock and unlockstates of braking near a slip rate providing the maximum frictioncoefficient of the road surface.

[0003] For this reason, it is a common practice for the conventional ABSto measure a speed of the vehicle and a revolution of the tire/wheelassembly, calculate the slip rate based on the measured values, andautomatically control the braking so as the slip rate to fall within acertain range.

[0004] However, the method for estimating the frictional coefficient ofthe road surface from the slip rate has a problem in which therelationship between the slip rate and the friction coefficient of theroad surface drastically changes depending on a road surface conditionto vary the slip rate corresponding to the optimum friction coefficientof the road surface depending on the road surface condition, so that theoptimum friction coefficient of the road surface cannot be obtained fromthe slip rate alone. Although approaches for solving this problem suchas separately estimating the road surface condition as well has beenmade, satisfactory means have not been proposed yet.

[0005] Meanwhile, in connection with this point, an approach for moredirectly measuring and estimating the friction coefficient of the roadsurface. Such an approach is known from the disclosure of JapanesePatent Application Opened No. 06-288798A. According to this disclosure,a strain gauge is attached to a suspension suspending the tire/wheelassembly and a strain occurring on this is measured to give a componentparallel to the friction force of the road surface or a componentperpendicular to the former component of a force acting on thesuspension. The means estimate the friction coefficient of the roadsurface based on these measured values by assuming the values as thefriction force of the road surface and the vertical force, respectively.

[0006] However, although the method with using a strain gauge is a moredirect estimating method as compared with the method of estimating thefriction coefficient with the slip rate, there are problems in which thepoint of measuring the force is far from a vicinity of tire which is theactual point of action of the friction force, so that a measuring resulthaving been influenced by disturbances applied between the point ofaction of the friction force and the point of measuring the force isobtained, and that the strain gauge is applied on the suspension inwhich a generated strain is small, and the generated strain is convertedto the force, so that its accuracy is not sufficient.

[0007] The present invention has been made in view of these problems. Itrelates to a method for directly estimating friction coefficient of aroad surface independently from the slip rate, and its object is toprovide a method of real-timely and more accurately estimating thefriction coefficient of the road surface by measuring the friction forceof the road surface and the vertical force at a region near the tire aswell as accurately measuring these forces, and a method and a device ofmultiplex transmission of signals upon transmitting a measurement signalof a force measured in the vicinity of tire to the ABS displaced on thevehicle body side.

DISCLOSURE OF THE INVENTION

[0008] The present invention has been completed to achieve theabove-mentioned object, and its gist, constitution and operation will bedescribed below.

[0009] (1)

[0010] A method of estimating friction coefficient of a road surfaceaccording to the present invention measures, in a tire/wheel assemblyhaving an axle hub, a tire, and a wheel attached to the axial hub tosupport the tire, tangential and vertical components of a force actingbetween a section of a transmission path between the axle hub and thetire and the other section of the transmission path, the sections of thetransmission path being bounded by an elastic body arranged within thetransmission path; and calculates the friction coefficient between thetire/wheel assembly and the road surface based on the measured values ofthe each component of the force.

[0011] According to the method of estimating the friction coefficient ofthe road surface, the friction coefficient is not calculated bymeasuring the slip rate and indirectly determining the frictioncoefficient of the road surface from the slip rate with a relationalexpression between the slip rate and the friction coefficient of theroad surface, but it is directly calculated by measuring the tangentialand vertical component of the force acting on a point of the vehicle.Therefore, the method can accurately and real-timely calculate thefriction coefficient which always changes depending on a condition ofthe road surface, so that an ABS superior in a braking performance canbe provided.

[0012] Further, it measures a force acting at a point on the vehicle ata point on the tire/wheel assembly being closest to a boundary betweenthe road surface and the assembly at which the friction force actuallyacts, so that the friction force can be more accurately determined,thereby contributing an improvement of a performance of the ABS.

[0013] (2)

[0014] The method of estimating the friction coefficient of the roadsurface according to the present invention is based on the methoddescribed in (1), wherein, in calculating the friction coefficientbetween the tire/wheel assembly and the road surface based on themeasurement values of the tangential and vertical components of theforce acting between the section of the transmission path and the othersection of the transmission path, an angular rate of the tire/wheelassembly is measured along with these measurements, and the frictioncoefficient μ between the tire/wheel assembly and the road surface iscalculated from the equation (1) with the measured tangential force Fb,vertical force N and angular rate ω as well as the known moment ofinertia I₀ and known effective radius R of the tire/wheel assembly.

μ=(Fb+(I ₀ /R)dω/dt)/N  (1)

[0015] By means of this method of estimating the friction coefficient ofthe road surface, the friction coefficient μ can be calculated by theequation (1). A ground for this equation is discussed with reference toFIG. 10. FIG. 10 is a side view of a wheel 1 and a tire 2 constituting apart of the tire/wheel assembly taken from a side. The tire/wheelassembly rotates at the angular rate ω. When the brake is applied on thetire/wheel assembly, the braking force Fb due to a braking torque Th,the friction force Fd with respect to the rotation, and a vertical forceN act on a part P displaced in a place at a distance of an effectiveradius R of the tire/wheel assembly from the rotational center.Regarding positive and negative directions of the force, the directionas shown in FIG. 10 by an arrow is defined a positive direction. Thedynamic equation of this part can be expressed by the equation (2) withthe moment of inertial being I₀. The relationship between the brakingtorque Tb and the braking force Fb can be expressed by the equation (3),and the relationship between the friction force and the vertical forceby the equation (4). In the equation (2), when the actual measured valueof the tangential force is converted to the braking torque, a radius rof the elastic body from the center of the tire/wheel assembly isassumed to be the same as the effective radius R of the tire/wheelassembly. However, if the radius r is different from the radius R, themeasured tangential force may be multiplied by (r/R) to obtain thebraking force.

I ₀·(dω/dt)=R·Fd−Th  (2)

Tb=R·Fb  (3)

Fd=μ·N  (4)

[0016] These equations (2), (3), and (4) can derive the equation (1).

[0017] (3)

[0018] The method of estimating the friction coefficient of the roadsurface according to the present invention is based on the methoddescribed in (1) or (2), wherein relative displacements of the sectionsof the transmission path in the tangential and vertical directions aremeasured, and the tangential and vertical components of the force actingbetween the sections of the transmission path are calculated based onthe displacements.

[0019] In order to obtain the tangential and vertical components of theforce acting between both parts of the tire/wheel assembly being boundedby the elastic body, it is easy to measure the strain of the elasticbody subjected to the action of the force. As the measurement of thestrain, a method of measuring a change in the electric resistance withthe strain gauge and a method of measuring relative displacements of thesections which apply forces with each other and sandwich the elasticbody can be recited by way of example. If the elastic body having ahigher elasticity, such as a rubber, is used, the latter method isprecise and advantageous. Therefore, the method of estimating thefriction coefficient of the road surface can accurately calculate thetangential and vertical components of the force acting between theabove-mentioned sections of the tire/wheel assembly.

[0020] (4)

[0021] The method of estimating the friction coefficient of the roadsurface according to the present invention is based on the methoddescribed in (3), wherein the tangential relative displacements aresummed among each of four pairs of corresponding points comprising fourpoints symmetrically arranged on the section of the transmission pathabout its axial center and, associated with these points, fourcorresponding points symmetrically arranged on the other section of thetransmission path about its axial center, and the tangential relativedisplacement of the sections of the transmission path is measured basedon the summed value.

[0022] By means of the method of estimating the friction coefficient ofthe road surface, the tangential relative displacements are summed amongeach of four pairs of corresponding points to calculate the tangentialrelative displacements of the sections of the transmission path, so thatthey can be calculated independently from the position of the point ofmeasurement even not in a steady state.

[0023] This is discussed with reference to FIG. 11. FIG. 11 is a planeview showing a disk 30 as an example of one section of the transmissionpath, and a rim 31 as an example of the other section of thetransmission path being bounded by the elastic body. The rim 31 isconnected with the disk 30 via the not-shown elastic body. In a statewhere an eccentric load does not exist, the rim 31 rotates along arotational direction Q about an axial center identical to an axialcenter O₁ of the disk 30 in conjunction with the disk 30. In FIG. 11, anaxial center O₂ of the rim 31 is shifted by an amount of eccentricity Ddue to a vertical reaction force T from the road surface.

[0024] In this method, one point P₁ of four points of measurementsymmetrically displaced about the axial center O₁ of the disk 30 and onepoint P₂ of four points of measurement symmetrically displaced about theaxial center O₂ of the rim 31 are made a pair, and then the tangentialcomponents of the displacement of each of the corresponding points P₁and P₂ of the pair are measured. Thereafter, the tangentialdisplacements of each of the corresponding points is similarlycalculated for the other not-shown three pairs.

[0025] All the point of measurement is displaced at a distance of aradius R from each axial center, and rotates about each axial center.The momentary rotation angle is designated as θ.

[0026] With respect to the shown pair of the points of measurement, thetangential displacement X₁ is calculated. When the disk 30 and the rim31 are not eccentric and additionally the tangential relativedisplacement is zero, the point P₂ of measurement on the rim 31 agreeswith the point P₁. When the tangential relative displacement a betweenthe disk 30 and the rim 31 occurs while remaining in the state where noeccentric exists, the point of measurement on the rim 31 is moved to thepoint P₂. The amount of displacement between the corresponding points isequal to the distance between the points P₁ and P₂, so that thetangential component X₁ is equal to the distance between the points P₁and P₃, which is expressed by the equation (5) as evidenced by thefigure.

X ₁ =r·sin (α)+D·cos (θ)  (5)

[0027] Although the tangential relative displacement α needs to becalculated, the momentarily-changing relative displacement α between thedisk 30 and the rim 31 cannot be calculated based on only one pair ofthe corresponding points, as is apparent from the equation. It isbecause the X₁ depends on the rotation angle θ of the point ofmeasurement. Consequently, the method of estimating the frictioncoefficient of the road surface enables a calculation of the tangentialrelative displacement α independently of the rotation position of a pairof the point of measurement by summing each of the tangentialdisplacements of the four pairs of the corresponding points, andcalculating the tangential relative displacement a based on the summedvalue Y₁, as shown in the equation (6). $\begin{matrix}\begin{matrix}{Y_{1} = {{r \cdot {\sin (\alpha)}} + {D \cdot {\cos (\theta)}} +}} \\{{{r \cdot {\sin (\alpha)}} + {D \cdot {\cos \left( {\theta + {\pi \text{/}2}} \right)}} +}} \\{{{r \cdot {\sin (\alpha)}} + {D \cdot {\cos \left( {\theta + \pi} \right)}} +}} \\{{{r \cdot {\sin (\alpha)}} + {D \cdot {\cos \left( {\theta + {3\pi \text{/}2}} \right)}}}} \\{= {4{r \cdot \sin}\quad (\alpha)}}\end{matrix} & (6)\end{matrix}$

[0028] (5)

[0029] The method of estimating the friction coefficient of the roadsurface according to the present invention is based on the methoddescribed in (3), wherein the radial relative displacements are measuredamong each of four pairs of corresponding points comprising four pointssymmetrically arranged on the section of the transmission path about itsaxial center and, associated with these points, four correspondingpoints symmetrically arranged on the other section of the transmissionpath about its axial center, the products of two pairs of the relativedisplacements in a diagonal relation among the relative displacement arerespectively calculated and summed, and the vertical relativedisplacement of the sections of the transmission path is measured basedon the summed value.

[0030] By means of this method of estimating the friction coefficient ofthe road surface, the products of the radial relative displacements ofeach of the four corresponding points are calculated, two pairs of thecorresponding points in a diagonal relation among the relativedisplacements, the products of the two pairs are summed, and thevertical relative displacement of the above-mentioned section and theother section of the transmission path of the tire/wheel assembly beingbounded by the elastic body is decided based on the summed value, sothat the displacement can be calculated independently from the positionof the point of measurement even not in a steady state.

[0031] This is discussed with reference to FIG. 12. FIG. 12 is a planarview showing a disk 30 as an example of one section of the transmissionpath, and a rim 31 as an example of the other section of thetransmission path being bounded by the elastic body. In FIG. 12, the rim31 is connected to the disk 30 via the not-shown elastic body. In astate where an eccentric load does not exist, the rim 31 rotates along arotational direction Q about an axial center identical to an axialcenter O₅ of the disk 30 in conjunction with the disk 30. In FIG. 12, anaxial center O₆ of the rim 31 is shifted by an amount of eccentricity Ddue to a vertical reaction force T from the road surface.

[0032] In this method, one point P₅ of four points of measurementsymmetrically displaced about the axial center O₅ of the disk 30 and onepoint P₆ of four points of measurement symmetrically displaced about theaxial center O₆ of the rim 31 are made a pair, and then the radialcomponents of the displacement of each of the corresponding points P₅and P₆ of the pair are measured. Thereafter, the radial displacements ofeach of the corresponding points are similarly calculated for the othernot-shown three pairs.

[0033] Each of the points of measurement rotates about each axial centerand its momentary rotation angle is designated as θ.

[0034] With respect to the shown pair of the points of measurement, theradial displacement X₂ is calculated. When the disk 30 and the rim 31are not eccentric, the point P₆′ of measurement on the rim 31 agreeswith the point P₅ and the displacement of them is zero.

[0035] When an eccentricity occurs in an amount of D, the point ofmeasurement on the rim 31 is moved to the point P₆. The amount ofdisplacement between the corresponding points is equal to the distancebetween the points P₅ and P₆, so that the tangential component X₂ isequal to the distance between the points P₅ and P₇ which is expressed bythe equation (7) as evidenced by the figure.

X ₂ =D·sin (θ)  (7)

[0036] Although the amount of eccentricity D needs to be calculated, themomentarily-changing amount of eccentricity D cannot be calculated basedon only one pair of the corresponding points, as is apparent from theequation. It is because the X₂ depends on the rotation angle θ of thepoint of measurement. Consequently, the method of estimating thefriction coefficient of the road surface enables a calculation of theamount of eccentricity D independently of the rotation position of apair of the point of measurement by measuring each of the radialdisplacements of the four pairs of the corresponding points, calculatingthe products of the two relative displacements in a diagonal relationamong the relative displacements, and calculating the amount ofeccentricity D based on the summed value Y₂, as shown in the equation(8). $\begin{matrix}\begin{matrix}{{Y2} = {\left( {\left( {D \cdot {\sin (\theta)}} \right) \cdot \left( {D \cdot {\sin \left( {\theta + \pi} \right)}} \right)} \right) +}} \\{\left( {\left( {D \cdot {\sin \left( {\theta + {\pi/2}} \right)}} \right) \cdot \left( {D \cdot {\sin \left( {\theta + {3{\pi/2}}} \right)}} \right)} \right)} \\{= {{- 2}D^{2}}}\end{matrix} & (8)\end{matrix}$

[0037] (6)

[0038] The method of estimating the friction coefficient of the roadsurface according to the present invention is based on the methoddescribed in any one of (3) to (5), wherein a hall element is used tomeasure a change in magnetic flux density of the magnetic body, and thetangential or radial relative displacement is detected based on theamount of the change.

[0039] By means of this method of estimating the friction coefficient ofthe road surface, a hall element is used to measure the change inmagnetic flux density of the magnetic body to detect the tangential orradial relative displacement, so that the friction coefficient of theroad surface can be readily and accurately measured.

[0040] (7)

[0041] The method of multiplex transmission of signals according to thepresent invention, upon transmitting signals of the measured valuesdescribed in any one of (1) to (6) or other signals between the vehiclebody side and the rotating tire/wheel assembly side, a composite signalobtained by a voltage/frequency conversion of a plurality of signals isapplied to transmitting coil while switching the signals to transmit toa receiving coil by an electromagnetic induction, said plurality ofsignals having dynamic ranges without overlapping with each other, aplurality of signals obtained by a frequency/voltage conversion of thecomposite signal received by the receiving coil is processed to generatetiming signals with a plurality of given voltage levels as threshold,and said composite signal obtained by the frequency/voltage conversionis sampled according to said timing signals to reproduce a plurality ofthe original signals.

[0042] The above-mentioned measured values described in any one of (1)to (6) are measured by sensors provided on the rotating tire/wheelassembly side. The measured values need to be transmitted at least to anABS which controls a braking with using the measured values. Therefore,a signal transmittance between the rotating tire/wheel assembly side andthe vehicle body side provided with the ABS is needed.

[0043] For such a method of transmitting signals between a rotating partand non-rotating part, a method of transmitting signals on carrierwaves, so-called a wireless method and a wired method can be used.However, the former has problems in a complicated configuration as wellas a large electric power consumption, and the latter, which transmitsthe signals between the rotating part and the non-rotating part via amechanical contact point such as a slip ring so that the mechanicalcontact point extremely deteriorates by wear, has a problem in amaintenance.

[0044] As the third method, there has been suggested a method ofdisplacing a transmitting coil on the wheel of the tire side as well asdisplacing a receiving coil on the vehicle body side to face thetransmitting coil, and transmitting a detecting signal from a sensordisplaced on the tire side to the receiver coil by the action ofelectromagnetic induction. However, such a method of transmittingsignals by the action of electromagnetic induction is not configured tomultiplex a plurality of signals.

[0045] By means of the system of multiplex transmission of signalsaccording to the present invention, for example, when theabove-mentioned relative displacements are measured while the vehicle isrunning, and the measured signals are transmitted to the vehicle bodyside, the detected signals output from a plurality of sensors displacedon the tire side are converted into signals having dynamic rangeswithout overlapping with each other. Then, as mentioned above, avoltage/frequency conversion is conducted to generate a composite signalwhich is transmitted from the transmitting coil to the receiving coil bythe action of electromagnetic induction. The composite signal receivedby the receiving coil is subjected to the frequency/voltage conversion,and a plurality of timing signals are generated with a plurality ofgiven voltage level as thresholds. The composite signal obtained by thefrequency/voltage conversion is sampled/held according to the timingsignals to be able to reproduce a plurality of the original signals.

[0046] (8)

[0047] The device of multiplex transmission of signals according to thepresent invention is used for the method of multiplex transmission ofsignals described in (7), which comprises:

[0048] a signal-generating circuit for generating a plurality of signalshaving dynamic ranges without overlapping with each other; a switchingcircuit for outputting a composite signal by sequentially switching theplurality of signals simultaneously output from the signal-generatingcircuit at a given cycle; a voltage/frequency conversion circuit forconverting the composite signal output from the switching circuit into acomposite signal having a frequency corresponding to the voltage; anoutput circuit for amplifying the composite signal output from thevoltage/frequency conversion circuit; a transmitter coil fortransmitting the amplified composite signal output from the outputcircuit; a receiver coil for receiving the composite signal transmittedfrom the transmitter coil by the action of electromagnetic induction; afrequency/voltage conversion circuit for converting the composite signalreceived by the receiver coil into a composite signal having anamplitude corresponding to the frequency; a timing signal-generatingcircuit for generating a plurality of timing signals by processing thecomposite signal output from the frequency/voltage conversion circuitwith a plurality of given voltage levels as thresholds; a plurality ofsample/hold circuits for sampling the composite signal output from thefrequency/voltage conversion circuit according to the timing signals toreproduce a plurality of the original signals.

[0049] By means of the device of multiplex transmission of signalsaccording to the present invention, as is apparent from itsconfiguration, a plurality of the signals described in (7) can bemultiplexed and the multiplexed signals can be transmitted between thetire/wheel assembly side and the vehicle body side.

BRIEF DESCRIPTION OF DRAWINGS

[0050]FIG. 1 is a sectional view of the tire/wheel assembly showing anembodiment of the measurement method according to the present invention;

[0051]FIG. 2 is a side view of the tire/wheel assembly showing in astate where the tire/wheel assembly is rotating;

[0052]FIG. 3 is a side view of the tire/wheel assembly taken in adirection of the arrows III-III in FIG. 1;

[0053]FIG. 4 is a configuration view showing a configuration of thetangential and vertical displacement sensor bodies;

[0054]FIG. 5 is a block diagram of the circuit for computing the outputof the tangential displacement sensor body;

[0055]FIG. 6 is a block diagram of the circuit for computing the outputof the vertical displacement sensor body;

[0056]FIG. 7 is a block diagram showing the friction coefficientcomputing section;

[0057]FIG. 8 is a block diagram showing the whole configuration of thesignal multiple signal transmitting device;

[0058]FIG. 9 is a signal wave form chart for explaining the generationof the timing signals;

[0059]FIG. 10 is a side view of the wheel and the tire;

[0060]FIG. 11 is a plane view showing the disk and the rim connectedthereto;

[0061]FIG. 12 is a plane view showing the disk and the rim connectedthereto;

BEST MODE FOR CARRYING OUT THE INVENTION

[0062] Hereinafter, embodiments of the present invention will bedescribed with reference to FIGS. 1 to 12. FIG. 1 is a sectional view ofa wheel 1 showing in a state where the wheel 1 is attached to an axlehub 7 of a vehicle. The wheel 1 is provided with a disk 4 mounted to theaxle hub 7, a rim 2 supporting a tire 6, and a rubber-like elastic body3 connecting the disk 4 with the rim 2 and equipped in a part of aforce-transmission path between the axle hub 7 and the tire 6. Beingbounded by the rubber-like elastic body 3, one section of theforce-transmission path is composed of the disk 4, and the other sectionof the force-transmission path is composed of the rim 2. A tire/wheelassembly is consist of the axle hub 7, the tire 6 and the wheel 1.

[0063]FIG. 2 is a side view of the wheel showing in a state where thewheel 1 is mounted on a running vehicle and rolls along the roadsurface. The rim 2 connected to the disk 4 via the rubber-like elasticbody 3 is nearly concentric with the axial line of the disk 4. As therubber-like elastic body 3 deforms in the vertical direction due to avertical action of the gravity, the rim 2 is relatively displaced by anamount of eccentricity D in a direction T with respect to the disk 4.The rim 2 is driven via the rubber-like elastic body 3 by the disk 4 torotate in a rotational direction Q. In this context, the rim 2torsionally rotates with a tangential relative displacement α withrespect to the disk 4 due to an elastic deformation of the rubber-likeelastic body 3 in the rotational direction.

[0064]FIG. 3 is a side view of the wheel taken in a direction of thearrows III-III in FIG. 1. As shown in FIG. 3, tangential displacementsensor bodies 11 and vertical displacement sensor bodies 12 are providedat four points on a periphery of the boundary between the rim 2 and disk4 of the wheel 1.

[0065]FIG. 4 is a configuration view showing the configuration of thetangential displacement sensor body 11 and the vertical displacementsensor body 12. These sensor bodies 11 and 12 are composed of a barmagnet 15, a hall element probe 14 oppositely disposed on an axial lineL of the bar magnet 15, and an amplifier 16. These sensor bodies 11 and12 can detect changes in a gap between the tip of bar magnet 15 and thetip of the hall element probe 14 along the axial line.

[0066] Four bar magnets 15 constituting the four tangential displacementsensor bodies 11 are attached on the rim 2 to positions being symmetricwith respect to the axial center of the rim 2. Four hall element probes14 each corresponding to respective bar magnet 15 are attached on thedisk 4 to positions being symmetric with respect to the axial center ofthe disk 4. The axial line L is directed to the tangential line.

[0067] In this state, each pair composed of the tip of the bar magnet 15and the tip of the corresponding hall element probe 14 constitutes apaired corresponding point. Tangential relative displacements of thesecorresponding points are measured and the relative displacements ofthese four pairs are summed. A tangential displacement of the disk 4with respect to the rim 2 is calculated based on the summed value Y₁.That is, when a continuously-changing rotation angle of the wheel 1, adistance from the axial center of the corresponding points, an amount ofeccentricity, and the tangential relative displacement between the rim 2and the disk 4 are designated as θ, r, D, and α, respectively, thetangential relative displacement a can be calculated from the summedvalue Y₁ of the four pairs of the relative displacements according tothe above-mentioned equation (6). The value Y1 is independent from thecontinuously-changing rotation angle θ, so that the tangential relativedisplacement a can be real-timely calculated.

[0068]FIG. 5 is a block diagram of a circuit for computing an output ofthe tangential displacement sensor body 11. The outputs of amplifiers16A of the tangential displacement sensor body 11 are input to anaccumulator 21 to output a summed value Y₁.

[0069] In the next, a method of calculating a vertical displacement withrespect to the disk 4 of the rim 2, i.e. an amount of eccentricity isdescribed. Four bar magnets 15 constituting the four verticaldisplacement sensor bodies 12 are attached on the rim 2 to positionsbeing symmetric with respect to the axial center of the rim 2. Four hallelement probes 14 each corresponding to the respective bar magnets 15are attached on the disk 4 to positions being symmetric with respect tothe axial center of the disk 4. The axial line L is directed to theradial direction.

[0070] In this state, each pair composed of the tip of the bar magnet 15and the tip of the corresponding hall element probe 14 constitutes apaired corresponding point. Radial relative displacements of thesecorresponding points are measured and the products of two pairs of therelative displacements in diagonal relationships among the relativedisplacement are calculated. The two pairs of the products are summed tocalculate the amount of eccentricity based on the summed value Y₂. Thatis, when a continuously-changing rotation angle of the wheel 1 and theamount of eccentricity are designated as θ and D, respectively, theamount of eccentricity D can be calculated from the summed value Y₂according to the above-mentioned equation (6). The value Y₂ does notdepend on the continuously-changing rotation angle θ, so that that theamount of eccentricity D can be real-timely calculated as an amountindependent from the rotation angle θ.

[0071]FIG. 6 is a block diagram of a circuit for computing an output ofthe vertical displacement sensor body 12. The outputs from twomultiplier for multiplying the outputs of the amplifiers 16B of thevertical displacement sensor bodies 12 diagonally displaced each otherare summed by the accumulator 24 to out put as Y₂. Y₂ is computed by asoftware section 25 to inversely calculate a eccentricity value Y₃corresponding to the amount of eccentricity D according to the equation(8).

[0072]FIG. 7 is a block diagram showing a friction coefficient-computingsection 26 for calculating the friction coefficient. The frictioncoefficient-computing section 26 estimates and calculates the frictioncoefficient based on the summed value Y₁ from the tangentialdisplacement sensor body 11, the amount of eccentricity Y₃ from thevertical displacement sensor body 12, and an input value Z of angularrate from a not-shown rotational speed sensor of the tire/wheel assemblyaccording to the method of the above-mentioned equation (1).

[0073] The method of estimating the friction coefficient based on theoutput of each displacement sensor body has been described in the abovewith reference to the block diagrams shown in FIGS. 5-7. The amplifiers16A and 16B of each displacement sensor body, accumulators 21 and 24constituting the computing circuits for calculating the summed values Y₁and Y₂, and the multiplier 22 are attached to each of the rotatingtire/wheel assemblies and a computer section for calculating thefriction coefficient from the summed values Y₁ and Y₂, i.e. the softwaresection 25 and the friction coefficient-computing section 26 areequipped on the vehicle body side, so that the summed values Y₁ and Y₂have to be transmitted from the tire/wheel assembly side to the vehiclebody side, which is preferably not by means of signal lines. Because awired transmission can transmit the signals via an slip ring or the likebut it cannot avoid problems resulting from a wear or a heat due to aslip between the rotating section and the fixed section.

[0074] When the transmission is not by means of signal lines, it iswasteful to provide two transmission paths and transmit the summedvalues Y₁ and Y₂ from the tire/wheel assembly side to the vehicle bodyside by the paths, so that a multiplex transmission is preferable.Although transmitting signals by means of a radio wave is a commonmethod among the methods not by means of signal lines, such a signaltransmission from the tire/wheel assembly side to the vehicle body sideof the running vehicle tends to be affected by a noise, so that a systemof transmitting a signal by an electromagnetic induction is adopted.

[0075] With reference to FIGS. 8 and 9, a multiplex transmitting deviceused in this embodiment is described below. FIG. 8 is a block diagramshowing a whole configuration of the multiplex transmitting device 40.The multiplex transmitting device 40 transmits the summed value signalsY₁ and Y₂ from the tire/wheel assembly side to the vehicle body side.The method of transmitting signals from the tire/wheel assembly side tothe vehicle body side is discussed below. Original signals consisting ofthe summed values Y₁ and Y₂ are input to a signal generating circuit 29and are converted at the signal generating circuit 29 withoutoverlapping dynamic ranges with each over. As an example of the dynamicrange, for example, the summed values Y₁ are in the voltage range of0.5-1.0 V and Y₂ are in voltage ranges of 1.5-2.0V.

[0076] Then, the converted summed value signals are supplied to aswitching circuit 32 and the switching circuit 32 sequentially switchesand outputs the signals at a given cycle. In this specification, such asignal output from the switching circuit 32 is referred to as a“composite signal”. As the dynamic ranges of the summed value signals Y₁and Y₂ are different from each other, the signal jumps at the time ofswitching by the switching circuit 32.

[0077] The composite signal output from the switching circuit 32 in thismanner is then supplied to a voltage-frequency conversion circuit 33.The voltage-frequency conversion circuit 33 has a function of convertingthe input signal to the output signal with the frequency correspondingthe voltage, so that the voltage-frequency conversion circuit 33 outputsthe composite signals with their frequency ranges being sequentiallychanged. Further, after the composite signals output from thevoltage-frequency conversion circuit 33 are amplified by the outputcircuit 34, the amplified composite signal output from the outputcircuit 34 is supplied to a transmitting coil 35. The transmitting coil35 is displaced to be concentric with the axle hub 7 and rotatesintegrally with the tire 6 and the wheel 1.

[0078] On the vehicle body side of the vehicle, a receiving coil 36 isdisposed to face the above-mentioned transmitting coil 35 and isconfigured to receive the composite signals transmitted by theelectromagnetic induction from the transmitting coil 35. As mentionedabove, the amplifiers 16A and 16B of each displacement sensor body, thecomputing circuit including the accumulators 22 and 24, and multiplier23, the switching circuit 32, the voltage-frequency conversion circuit33, and the output circuit 34 as well as the transmitting coil 35 aredisposed on the tire/wheel assembly side. Electric power for them issupplied by the electromagnetic induction from the receiving coil 36 tothe transmitting coil 35, so that a power source such as a battery isnot needed. As supplying electric power by the electromagnetic inductionof the coil is known per se, no further detail is discussed here.

[0079] As mentioned above, the composite signal received by thereceiving coil 36 is supplied to the frequency/voltage conversioncircuit 37 to convert it to a composite signal having an amplitudecorresponding to the frequency. The composite signal output from thefrequency/voltage conversion circuit 37 contains mixed information ofmultiple signals, so that the signals need to be identified. To thisend, the composite signal output from the frequency/voltage conversioncircuit 37 is supplied to a timing signal generating circuit 38. Thetiming signal generating circuit 38 sets a plurality of given voltagelevels as the thresholds. As shown in FIG. 9, the voltage at eachboundary of the dynamic ranges, e.g. 1.2 V is set as the threshold.

[0080] In the case where the first signal S1 in the composite signalswitches to the second signal, the amplitude of the composite signalgoes across the threshold of 1.2 V, and in the case where the secondsignal S₂ in the composite signal switches to the first signal, theamplitude of the composite signal goes across inversely (from the largerregion to the smaller region). In this way, the amplitude of thecomposite signal goes across sequential thresholds at the switching ofthe sequential signals contained in the composite signal, so that thegiven signal can be appropriately sampled and held by sampling thesignal at a time of the given time τ having passed since each of thetiming signals was generated from the timing signal generating circuit38 at these timing.

[0081] In this manner, the plurality of the timing signals output formthe timing signal generating circuit 38 are respectively supplied to twosample/hold circuits 39 each receiving the composite signal output fromthe above-mentioned frequency/voltage conversion circuit 37. In thesesample/hold circuit, the composite signal is sampled and held accordingto the sampling signals generated at a time of the time X having passedsince each of the timing signals has generated, so that each of theoriginal signals contained in the composite signal can be reproduced. Inthis way, a plurality of signals can be multiplex by the electromagneticinduction with the transmitting coil 35 and receiving coil 36. Thesesignals are supplied to a signal processing circuit equipped in thevehicle body side where a given process is conducted to be able toestimate the friction coefficient.

[0082] In the above, a description is made to a case where theaccumulators 21, 24 and the multiplier 22 are equipped on the rotatingtire/wheel assembly. However, the accumulator 21, 24 and the multiplier24 may be equipped on the vehicle body side. In this case, total ofeight signals from the amplifiers 16A, 16B are input to the signalgenerating circuit 29 and the eight kinds of signals are converted intoeight signals having dynamic ranges different from each other.Thereafter, the signals are transmitted to the vehicle body side asmentioned above to reproduce the eight original signals.

[0083] Moreover, as mentioned above, in the method and device ofmultiplex transmission of the signals according to the presentinvention, operations similar to the sampling of each signal areconducted at the switching circuit 22, so that a switching frequency inthe switching circuit 22 can be determined based on the samplingtheorem. For example, when the highest frequency in a plurality of thesignals to be transmitted and the number of the signals are designatedas f_(H) and N, respectively, and each signal is transmitted at the samerate, the switching frequency f_(S) may be f_(S)=2Nf_(H).

[0084] Further, the present invention is not limited to theabove-mentioned examples, there may be a number of modifications andvariations. For example, in the above-mentioned example, the contiguousdynamic ranges of a plurality of the signals to be transmitted are set,but discontiguous dynamic ranges such as 0-1 V for the first signal and2-3V for the second signal may be set. Also, in the above-mentionedexample, all the transmitting rate of the signals are set to be equal bysequentially transmitting a plurality of the signals, but thetransmitting rate of each signal may be varied.

INDUSTRIAL APPLICABILITY

[0085] As having been clearly shown in the above description, accordingto the present invention, the tangential and vertical forces acting onthe elastic body on the elastic wheel are measured and the frictioncoefficient of the road surface is estimated based on the measuredforces, so that the friction coefficient of the road surface can bedirectly and accurately estimated while being independent from the sliprate, which contributing an improvement in a performance of the ABS.

[0086] Moreover, when the measured values measured at the tire/wheelassembly side are transmitted to the ABS on the vehicle body side, aplurality of signals are multiplexed and transmitted by the action ofelectromagnetic induction. Thus, a transmitting device being simple andhighly tolerant of noises can be configured. In addition, bymultiplexing the signals, the ABS capable of transmitting a number ofinformation within a given time and having a short response time can berealized.

[0087] Further, the method and device of multiplex transmission ofsignals are applicable not only for the transmission between therotating tire/wheel assembly and the vehicle body, but also for otherapplication of transmitting a number of signals without using signallines.

1. A method for estimating friction coefficient of a road surface,comprising measuring, in a tire/wheel assembly having an axle hub, atire, and a wheel attached to the axial hub to support the tire,tangential and vertical components of a force acting between a sectionof a power transmission path between the axle hub and the tire and theother section of the transmission path, said sections of thetransmission path being bounded by an elastic body arranged within thetransmission path; and calculating the friction coefficient between thetire/wheel assembly and the road surface based on the measured values ofthe each component of the force.
 2. The method for estimating frictioncoefficient of a road surface according to claim 1, wherein, incalculating the above-mentioned friction coefficient between thetire/wheel assembly and the road surface based on the measurement valuesof the tangential and vertical components of the force acting betweensaid section of the transmission path and said the other section of thetransmission path, an angular rate of the tire/wheel assembly ismeasured along with these measurements, and the friction coefficient μbetween the tire/wheel assembly and the road surface is calculated fromthe following equation (1) with the measured tangential force Fb,vertical force N and angular rate ω as well as the known moment ofinertia I₀ and known effective radius R of the tire/wheel assembly.μ=(Fb+(I ₀ /R)dω/dt)/N  (1)
 3. The method for estimating frictioncoefficient of a road surface according to claim 1 or 2, whereinrelative displacements of said section and said the other section of thetransmission path in the tangential and vertical directions aremeasured, and the tangential and vertical components of the force actingbetween said section and said the other section of the transmission pathare calculated based on the measured value.
 4. The method for estimatingfriction coefficient of a road surface according to claim 3, wherein thetangential relative displacements are summed among each of four pairs ofpoints comprising four points symmetrically arranged on said section ofthe transmission path about its axial center and, associated with thesepoints, four corresponding points symmetrically arranged on said theother section of the transmission path about its axial center, and thetangential relative displacement of said section and said the othersection of the transmission path is determined based on the summedvalue.
 5. The method for estimating friction coefficient of a roadsurface according to claim 3, wherein the radial relative displacementsare measured among each of four pairs of points comprising four pointssymmetrically arranged on said section of the transmission path aboutits axial center and, associated with these points, four correspondingpoints symmetrically arranged on said the other section of thetransmission path about its axial center, the products of two pairs ofthe relative displacements in a diagonal relation among the relativedisplacement are respectively calculated and summed, and the verticalrelative displacement of said section and said the other section of thetransmission path is determined based on the summed value.
 6. The methodfor estimating friction coefficient of a road surface according to anyone of claims 3 to 5, wherein a hall element is used to measure a changein magnetic flux density of a magnetic body, and the tangential orradial relative displacement is detected based on the amount of thechange.
 7. A method of a multiplex transmission of signals,characterized in that, upon transmitting signals of the measured valuesrecited in any one of claims 1 to 6, or other signals, between thevehicle body side and the rotating tire/wheel assembly side, a compositesignal obtained by a voltage/frequency conversion of a plurality ofsignals is supplied to a transmitting coil while switching the signalsto transmit to a receiving coil by an electromagnetic induction, saidplurality of signals having dynamic ranges without overlapping with eachother, a plurality of signals obtained by a frequency/voltage conversionof the composite signal received by the receiving coil is processed togenerate timing signals with a plurality of given voltage levels asthresholds, and said composite signal obtained by the frequency/voltageconversion is sampled according to said timing signals to reproduce aplurality of the original signals.
 8. A device of multiplex transmissionof signals for carrying out the method of multiplex transmission ofsignals according to claim 7, comprising: a signal-generating circuitfor generating a plurality of signals having dynamic ranges withoutoverlapping with each other; a switching circuit for outputting acomposite signal by sequentially switching the plurality of signalssimultaneously output from the signal-generating circuit at a givencycle; a voltage/frequency conversion circuit for converting thecomposite signal output from the switching circuit into a compositesignal having a frequency corresponding to the voltage; an outputcircuit for amplifying the composite signal output from thevoltage/frequency conversion circuit; a transmitter coil fortransmitting the amplified composite signal output from the outputcircuit; a receiver coil for receiving the composite signal transmittedfrom the transmitter coil by the action of electromagnetic induction; afrequency/voltage conversion circuit for converting the composite signalreceived by the receiver coil into a composite signal having anamplitude corresponding to the frequency; a timing signal-generatingcircuit for generating a plurality of timing signals by processing thecomposite signal output from the frequency/voltage conversion circuitwith a plurality of given voltage levels as thresholds; a plurality ofsample/hold circuits for sampling the composite signal output from thefrequency/voltage conversion circuit according to the timing signals toreproduce a plurality of the original signals.